Recent governmental regulations have lowered the specification for sulfur concentration in gasoline. A significant source of sulfur in gasoline is from cracked naphtha which is a major refinery blending component of gasoline. In fact, cracked naphtha, in some instances, may account for as much as 75 percent of the total volume of gasoline produced by a given refinery. Typically, the cracked naphtha produced by a refinery accounts for as much as 25 to 50 volume percent of the refinery's total gasoline production.
Cracked naphtha can be either a catalytically cracked or thermally cracked product, and it typically has a high concentration of olefin and aromatic compounds that contribute to its relatively high octane property. The cracked naphtha further may have a high concentration of undesirable organic sulfur compounds, such as, mercaptan, organic sulfide (e.g. thioether), thiophene and heterocyclic sulfur compounds. The concentration of organic sulfur compounds in cracked naphtha may even be as high as up to 2 wt. % (20,000 ppmw), but, typically, it is in the range of from 500 ppmw to 15,000 ppmw. The concentration of desirable olefins contained in cracked naphtha, as noted, is also quite high and can be in the range of from 30 wt. % upwardly to 75 or 80 wt. % of a cracked naphtha stream.
One common method of removing sulfur compounds from hydrocarbon feedstocks is by hydrodesulfurization. This is done by passing the hydrocarbon feedstock over a hydrogenation catalyst in the presence of hydrogen and under suitable hydrodesulfurization reaction conditions so as to hydrogenate the organic sulfur compounds and convert the sulfur to hydrogen sulfide that can easily be removed from the hydrocarbons.
A disadvantage of using hydrodesulfurization to remove organic sulfur from cracked feedstocks, however, is that it also tends to hydrogenate the olefins contained in the cracked feedstock to alkanes. This is not typically desired because olefins generally have higher octane values than alkanes, thus, making it undesirable to saturate the olefins due to the lower economic value of the resulting product as a consequence of the reduced octane value of the treated product.
The prior art discloses various processes that address some of the noted problems associated with the use of hydrodesulfurization to remove organic sulfur from cracked feedstocks. One such process is disclosed in U.S. Pat. No. 6,946,068. This process includes two-stages for the desulfurization of a full-range cracked naphtha feed. The two-stage process uses a first catalytic distillation column followed by a second catalytic distillation column. The first catalytic distillation column contains two reaction zones. The first reaction zone provides for reacting thiophene with hydrogen to produce n-butyl mercaptan and the second reaction zone provides for reacting mercaptans with diolefins to produce sulfides. The first catalytic distillation column further provides for separating the lower boiling and the higher boiling portions of the feed. Because the formed sulfides tend to be higher boiling compounds, they generally pass from the first catalytic distillation column with the higher boiling bottoms product to the second catalytic distillation column. The second catalytic distillation column includes a hydrodesulfurization reaction zone for converting organic sulfur compounds, e.g. sulfides, to hydrogen sulfide by hydrogenation. The second catalytic distillation column further provides for separating the bottoms product taken from the first catalytic distillation column into an intermediate naphtha product and a heavy naphtha product.
Another prior art process for treating cracked feedstocks to remove organic sulfur is detailed in U.S. Pat. No. 8,628,656. This process uses two stages that are similar to and operate in a similar way to those described in U.S. Pat. No. 6,946,068. But, additionally, the process of U.S. Pat. No. 8,628,656 further includes the use of two stripper fractionator columns and an intermediate fixed-bed, single-pass reactor. The stripper fractionator columns provide for the separation of unreacted hydrogen and hydrogen sulfide from naphtha to provide a bottoms fraction from the each stripper. The intermediate fixed-bed, single-pass reactor provides for incremental hydrodesulfurization of the bottoms fraction passing from the first stripper fractionator column with the reactor effluent passing as a feed to the second stripper fractionator column.
U.S. Pat. No. 8,628,656 and U.S. Pat. No. 6,946,068 are both incorporated herein by reference.
While prior art processes are able to provide various levels of selective desulfurization of cracked naphtha streams, due to new, more stringent gasoline sulfur specifications, new or improved processes for treating cracked naphtha are needed in order to reach the lower sulfur concentrations required for gasoline but without significant octane loss caused by undesirable hydrogenation of the high octane olefin compounds of the cracked naphtha.